1. Field of the Invention
The invention relates to laser-optical digital data storage and retrieval. More particularly, the invention relates to three dimensional laser-optical digital data storage and retrieval media and to methods and apparatus for storing and retrieving data using such media.
2. State of the Art
The first digital computers manipulated minute quantities of information by today's standards. Information was digitally stored on paper cards in quantities on the order of several bytes per card or on tape in quantities on the order of several kilobytes. Today, the ubiquitous desktop computer typically has access to several hundred megabytes of storage in a relatively small package, either a magnetic or optical disc. In some commercial applications it is not uncommon to provide several hundred gigabytes of online data stored on magnetic storage RAID (redundant array of inexpensive discs) or several hundred megabytes of online storage being selected from an optical storage "jukebox" containing several hundred gigabytes of storage. Nevertheless, in the present "information age", the amount of data which is desired, or often required, to be made available online is increasing faster than the technology for providing compact storage of this data.
In addition to the general commercial trend toward providing vast amounts of digitally stored data for business and scientific as well as for personal use, governments have become even more desirous of storing huge quantities of data which can be quickly and efficiently accessed by computers at remote locations. Of particular interest recently is the centralized storage of health care information. If the United States should adopt a centralized health care system, it would be desirable or even necessary to provide an electronically accessible central storage for all the medical information for every citizen. Given the nature of medical records and the present population of the United States, such a central storage could encompass approximately 2.4.times.10.sup.18 bytes (2,400,000 terabytes) of data.
For example, the average number of x-ray films in the medical records of an American adult is thirty-eight 14.times.16 inch films. This assumes one major surgery (ten films), one minor surgery (four films), one accident (two films), and dental X-rays which may, over the course of a lifetime, amount to the equivalent of five 14.times.16 films. In addition, women are encouraged to have five mammograms prior to age forty and one mammogram every year from ages forty to sixty-five. The average number of films for an American male is thus twenty-three over the course of his life and the average number of films for an American female is thus fifty-three over the course of her life; the average number of x-ray films for all Americans therefore being thirty-eight. Ideally, X-ray films stored in digital form should be provided with relatively high resolution and broad gray scale or range of colors. An acceptable resolution for medical images could be 600 dpi (dots per inch) with a pixel depth of 24 bits. With this resolution and pixel depth, each 14.times.16 X-ray film would require 2.42.times.10.sup.8 bytes (242 megabytes). In addition to X-ray films, the average medical records for an adult will likely contain an additional 4 megabytes of other information including text, sound, line art graphics, and DNA data. Therefore, a complete medical history in digital form for an average adult will likely require approximately 9200 megabytes (4 megabytes+[242 megabytes.times.38]).
Even using the best technology available, managing electronic access to that amount of data for 260 million people (9.2.multidot.10.sup.9 .times.2.6.multidot.10.sup.8 .apprxeq..multidot.2.4.multidot.10.sup.18 bytes) would be cumbersome, incredibly expensive, and would require a very large amount of physical space. In fact, real-time or even timely access to the specific records would be generally impossible given today's storage and retrieval technology.
Laser disc technology presently provides one of the most space efficient, as well as cost efficient, digital storage media. Compact laser discs are manufactured from a sandwich of inexpensive plastic which is depicted schematically in prior art FIG. 1. An inner substrate 10 of optically reflective material approximately 0.5 .mu.m thick is held between two layers of clear plastic 12, 14 each approximately 1.1 mm thick. The inner substrate is marked with microscopic pits 16 and flats 18 (each approximately 4 .mu.m square and differing in depth by as little as 0.1 .mu.m) which represent binary data. The pits and flats are arranged in concentric circular tracks 20. The disc is read by a photo detector and a laser beam having a diameter of approximately 0.1 .mu.m which are aimed at the tracks and moved radially from track to track while the disc is spun at high speed.
Prior art FIG. 2 shows a schematic diagram of a laser disc reading apparatus. A drive motor 22 spins the disc 24 at approximately 200-500 rpm. An optical reader 26 which includes a laser 28, a prism 30, a lens system 32, and a photo detector 34 is aimed at the disc 24 and is moved mechanically from one track to the next. Since the data containing surface of the disc is planar, the optical reader need only move in a single linear path. It has been contemplated, however, to provide an optical reader which moves in a curvilinear path. For example, U.S. Pat. No. 4,995,025 to Schultze, the complete disclosure of which is incorporated herein by reference, discloses a "Spherical Pivoting Actuator for Read/Record Head". Various methods are used to acquire and maintain focus of the optical reader relative to the disc such as that disclosed in U.S. Pat. No. 4,677,605 to Abed for "Focus acquisition and Maintenance for Optical Disk System", the complete disclosure of which is incorporated herein by reference.
Referring, once again, to prior art FIG. 2, the drive motor 22 changes speed as different tracks are read to maintain a constant linear speed of the data containing tracks relative to the optical reader 26. Light from the laser 28 is reflected from the flats on the accessed disc track and is received through the lens system 32 and directed by the prism 30 to the photo detector 34. Some systems have proposed elimination of the prism such as the system disclosed in U.S. Pat. No. 4,771,415 to Taki for an "Optical Data Storage and Readout Apparatus and Head, Using Optical Fibers Between Stationary and Movable Units", the complete disclosure of which is incorporated herein by reference. In any case, as the laser light is more or less reflected from the flats and pits, the detector 34 senses a sequence of relative lightness and darkness representing digital 1's (flats) and 0's (pits). These 1's and 0's represent the digital data stored on the disc.
Most compact laser discs are Read Only media (CDROMs) and are manufactured by pressing the substrate with a die much the same way as vinyl phonograph records are produced. However, recent technology has made it possible to provide laser discs which can be written on and erased as well as read. This technology involves the physical nature of the substrate which can be deformed and reformed in response to different types of laser radiation. Rewritable optical discs are called phase-change optical discs, and they are coated with a thin, crystalline film. When a laser beam having a first intensity strikes the film, it forms glasslike, or amorphous, spots that change the film's reflectivity thereby simulating the pits and flats of a CDROM and thus "recording" data on the disc. A lower level laser beam is used to read the discs in a substantially conventional manner. A laser beam having a second intensity can restore the crystalline structure of the film to its original state thereby "erasing" the spots which were recorded with the first intensity laser beam.
Compact laser discs range in diameter from two inches to sixteen inches. A popular "five inch" disc, which has a capacity of approximately 680 megabytes, is actually approximately 4.73 inches in diameter and is approximately 0.05 inches high (thick). It has a central mounting hole surrounded by a clear unusable area having a diameter of approximately 1.6 inches. The usable area upon which data may be stored on the disc is approximately 15.3 square inches. The five inch compact disc therefore represents an areal storage density of approximately 44.4 megabytes per square inch. Each compact disc occupies a volume of approximately 0.88 cubic inches using the formula V=.pi.r.sup.2 h. A protective caddy or jewel box case for the disc is typically approximately 5.6 inches by 4.9 inches by 0.4 inches deep and therefore has a volume of approximately 11 cubic inches. In its protective caddy, the "five inch" disc represents a volumetric storage density of approximately 62 Mb/cu.in. While this is an impressive number, it could be much higher if the protective caddy and the two layers of clear plastic could be removed. Unfortunately, such removal would render the disc so fragile as to make it unreliable. Even with the two layers of clear plastic, the disc is not invulnerable to fatal scratches. When a disc is being constantly handled, such as in a CD jukebox, it should be protected from dirt and scratches by a disc caddy.
Given a storage capacity of 680 megabytes per disc, it would require 13.5 compact discs (9200 megabytes divided by 680 megabytes) to store the medical records for a single person. For a population of 260 million, approximately 3.5.times.10.sup.9 compact discs would be required to store complete medical information for everyone. Given that the protected volume of a compact disc in a jewel box or caddy is approximately 11 cubic inches, a space of 3.86.times.10.sup.10 in.sup.3 or 2.23.times.10.sup.7 ft.sup.3 would be required to house enough five inch compact discs to store the medical records of every person in the United States. This space can be exemplified by a forty-seven story building the size of a football field (300 feet by 160 feet). Such a building would only provide the space necessary to house the discs packed as closely together as possible. Clearly, a much larger building would be required to provide convenient access to the discs. In addition, no matter how large the building, convenient access to three and a half billion compact discs seems unlikely no matter what method were used.
It is possible to store almost 39 gigabytes of data on a single sixteen inch optical disc, almost sixty times the storage capacity of a compact five inch disc. However, a sixteen inch optical disc housed in a protective case which is 16.2 inches square and 0.4 inches thick occupies a volume almost ten times that of a compact disc. Therefore, while some considerable saving in space can be achieved through the use of sixteen inch discs, the number of discs required will still be very high (on the order of tens of millions) and the storage space required will still be on the order of hundreds of thousands of cubic feet.
Recently introduced CDROM jukebox systems attempt to provide large amounts of online storage, but the best of these systems provides only about 65 gigabytes. For example, the CASCADE CD-100 from PINNACLE MICRO of California includes a TOSHIBA double-speed CDROM drive and an apparatus for shuffling one hundred conventional CDROM discs in which a disc is retrieved in approximately six seconds from when it is requested. Another jukebox system called PRAXIS from MAXOPTIX (Boston, Mass.) utilizes two 1.3 gigabyte erasable optical disc drives and forty-six optical cartridges held in an auto-changer for a total Online capacity of about 60 gigabytes. The auto-changer is alleged to load an optical cartridge in about 2.5 seconds.
Apart from jukeboxes, recent developments in optical storage technology have been aimed at increasing storage capacity of individual discs through a number of different techniques. In May 1994, SONY disclosed a new storage technique which substantially increases the capacity of an optical disc. See, SCIENTIFIC AMERICAN, August 1994, Volume 271, Number 2, Page 87. In conventional optical discs, information is stored by varying the length of each pit and the distances between the pits (i.e. the length of the flats and the pits). The SONY system arranges the centers of the pits at regular intervals with respect to one another while varying the distance from the front and rear edge of each pit to its center. This scheme doubles the number of bits that each pit holds.
TOSHIBA recently developed a 3.5-inch, double-sided disc which holds 606 megabytes. Late last year MATSUSHITA introduced a 5.25-inch phase-change disc having a capacity of 1.5 gigabytes. In May 1994 the company announced a method, similar to SONY'S edge-modulation technique, that could quadruple the capacity of a phase-change disc to 6 gigabytes. Id.
Efforts to increase the storage density of optical discs have also involved the type of laser used to record and read the pits and flats on an optical disc. Conventional optical storage systems use a red laser which can inscribe or read a pit which is approximately 4 .mu.m square. Experiments with blue-violet lasers demonstrate the ability to inscribe and read pits which are two and a half to three times smaller than the conventional pits. See, SCIENTIFIC AMERICAN, July 1994, Volume 271, Number 1, Page 100. However, these lasers, which require sophisticated cooling and are very expensive, are not yet commercially available and may not be for several years.
As the prior art demonstrates, the efforts made toward increasing the capacity of optical discs have been mainly aimed at optimizing the way data is stored on the discs (the size and dimensions of the pits as well as various data compression schemes) rather than at changing the overall geometry of the medium itself. Nevertheless, since the discovery of the hologram, thought has been given to methods of storing digital data in three dimensions. The IBM Almaden Research Center presently maintains a project to study the feasibility of holographic storage. See, SCIENTIFIC AMERICAN, October 1994, Volume 271, Number 4, Page 128. A technique developed by IBM multiplies the capacity of discs by stacking them on top of one another and gluing them together. Unlike conventional compact discs, which have opaque metal film substrates, the IBM optical discs are virtually transparent, allowing the laser to penetrate them and scan the surface of discs below. At the same time, the surfaces are reflective enough so that when the laser focuses on them, it can read the pits and flats. A movable lens adjusts the focal point of the laser, allowing it to read the surface of any disc in the stack. So far the IBM researchers have managed to place as many as six discs in a single stack and report that there are no technical obstacles to creating sandwiches of 10 discs, which would be only slightly thicker than a standard compact disc. This multiple surface optical storage system is described, at least in part, in U.S. Pat. No. 5,202,875 to Rosen et al. which is hereby incorporated herein by reference in its entirety. Some of the IBM-developed technology has been adopted by TAMARACK STORAGE DEVICES of Austin, Tex. Their holographic storage device, which is shown in prior art FIG. 3, uses a rotating disc 36 which is coated with a photopolymer. Holographic images as shown at 38 of pits and flats are created on the photopolymer 36 and read with two interfering laser beams 40, 42 which are derived from a single laser 44 with the use of a beam splitter 46, a mirror 47, a mirroring LCD 48, and a lens 49. The TAMARACK system uses 2.5 inch discs each of which is said to have the same capacity as a 5 inch CDROM. However, since the data is stored in a conventional photochemical manner on the photopolymer, it cannot be erased and rewritten.